1. Field of the Invention
This invention concerns extraction of metalloids and metals from solids and liquids, and is more particularly directed to a waste treatment process in which metals such as lanthanides and actinides are efficiently extracted from waste material.
2. General Discussion of the Background
Waste treatment and disposal is an important social and economic issue. Industries throughout the world spend large sums of money to reduce the biological hazards of environmental exposure to toxic substances. One particular environmental problem is the removal of toxic metals and radioisotopes from solid or liquid industrial wastes. Such contaminants can be removed from soils, for example, by treating the soil with an acid that dissolves the metals. Acid dissolution is followed by selective precipitation, electrowinning, or solvent extraction. Acid dissolution is unfortunately very nonspecific, and often produces many by-products that can create serious environmental problems in their own right.
An alternative detoxification process is to encapsulate contaminants in a container or insoluble matrix that prevents their entry into the environment. This approach still requires storage of the bulky matrix, and does not allow regeneration or reuse of the contaminants. Hence there is a need for a biologically compatible waste treatment process that efficiently and effectively separates metals from contaminated materials. There is also a need for such a process that is biologically compatible and permits selective regeneration and reuse of the contaminants.
One of the present inventors has previously disclosed that enhanced extraction of metals can be achieved with ionizable crown ethers, such as crown ether carboxylic acids. The inventors have found that these macrocyclic ethers have cavities that can selectively extract lanthanides and actinides by attracting these species with an ionized side chain. The metal ion is then inserted into the cavity of the macrocycle to form a chelate. Analyst, 114:451-453 (1989) and Anal. Chem 58:3233-3235 (1986). This mechanism of attracting the ion and inserting it in the ring has earned these compounds the name of "lariat crown ethers." In these reports, a crown ether carboxylic acid (sym-dibenzo-16-crown-5-oxyacetic acid) was used to extract lanthanides from aqueous solutions into an organic phase with high efficiency and selectivity. U.S. Pat. No. 4,908,135 similarly discloses separation of secondary and tertiary amines using a different crown ether, while U.S. Pat. No. 4,942,149 shows separation of racemic compounds with yet other crown ethers.
An unrelated solvent extraction method is supercritical fluid extraction. A supercritical fluid is typically one that is gaseous at ambient conditions, but which is maintained at a temperature and pressure above its critical temperature and pressure. Under those conditions it forms a solvent that can be used to extract organic materials such as caffeine from coffee beans. U.S. Pat. No. 4,911,941 provides an example of supercritical carbon dioxide extraction of caffeine in which green coffee beans are moved periodically through an extraction vessel and contacted with continuously flowing supercritical carbon dioxide. U.S. Pat. No. 4,898,673 shows a similar system in which soluble materials are continuously extracted from solids using supercritical carbon dioxide. The soluble solids are circulated in a closed loop pipeline with the supercritical fluid.
Supercritical extraction of environmental wastes has not previously been suggested. This may be due to the relatively low solubility of metals and other non-organic materials in supercritical fluids. Direct extraction of metal ions by supercritical carbon dioxide, for example, is inefficient because of the weak van der Waals interaction between metal ions and carbon dioxide. This weak interaction has apparently discouraged efforts to perform supercritical fluid extraction of metals from environmental wastes.
It is accordingly an object of this invention to provide an improved method for extracting metals from liquids or solids, including complex matrices.
It is another object of the invention to provide such an improved method that allows efficient and biologically compatible extraction of metals from the environment.
Yet another object is to provide such an improved method that allows selectivity as to the type of metal extracted by the system.
Even yet another object is to provide such an improved method that can selectively extract lanthanides and actinides.
Finally, it is an object of this invention to provide such an improved method that is efficient and economical compared to many other extraction processes.
These and other objects of the invention will be understood more clearly by reference to the following drawings and detailed description.